Spin label studies have shown that purple membranes from Halobacterium halobium are extremely rigid structures. Two populations of phospholipids were detected, one of which was more highly immobilied than the other. The highly immobilized lipid is probably boundary lipid which is tightly bound to bacteriorhodopsin, the only protein component of these membranes. Crosslinking of bacteriorhodopsin with glutaraladehyde caused a slight but significant increase in the fluidity of the membranes. No change in the molecular organization of purple membranes could be detected upon illumination with visible light.